The present invention concerns a method for preventing anode passivation in the electrolytic refining of copper, when periodical current reversal technology (PCR) is applied in electrolysis. The method is particularly suitable where the electrolytic refining of copper is carried out at high current densities. Irregular periodical current reversal technology is applied in the method, whereupon current reversal is regulated on the basis of an increase in voltage in the electrolysis cell.
In the electrolytic refining of copper, the impure so-called anode copper is dissolved with the aid of an electrical current and the dissolved copper is reduced onto cathode plates as totally pure so-called cathode copper. The electrolyte used is a sulfuric acid-based copper sulfate solution. A copper starting sheet, or so-called permanent cathode is used as the cathode plate at the beginning of the process; this permanent cathode can be made of acid resistant steel or titanium. One or more rectifiers are used as the power source in the electrolysis. As a rule, the current densities used in electrolysis are 250-320 A/m2 and the current is direct current (DC). Electrolysis takes place in separate electrolysis cells, where the number of anode-cathode pairs varies according to the plant, but is typically between 30 and 60 pairs. There are varying numbers of electrolysis cells in the plants. A typical anode dissolution takes from 14-21 days, the cathode cycle being 7-10 days.
The production capacity of the electrolysis plant is dependent on the amperage applied in electrolysis, on the number of electrolysis cells and on the time and current efficiency. The efficiencies describe how well in terms of time the cells in the plant are in use (by current) and how efficiently the electric current is used in the precipitation of the copper. The capacity of the electrolysis plant is increased by raising the current density, by building more electrolysis cells or by improving efficiencies.
Anode passivation sets the limit for the increase in current density, especially when using direct current (DC). During passivation, an insoluble layer forms on the surface of the anodes which layer hinders the dissolution reaction. In addition to the current density applied, the composition and the temperature of the electrolyte and in particular the composition of the anodes, all have an effect on passivation. Anode passivation causes losses in cathode copper production, increases energy consumption in the electrolysis process and impairs the quality of cathode copper.
Periodic current reversal technology (PCR), as described e.g. in GB patent 1157686 and in U.S. Pat. No. 4,140,596, is used in some copper electrolyses to raise cathode production capacity. The technology enables the current density applied to be raised in comparison to traditional direct current (DC) electrolysis.
Periodic reversal current technology is based on the reversal of the electrolytic current periodically. In this case the electrolysis is run for a certain time period (plus pulse, forward pulse) so that the copper dissolves from the anodes and precipitates onto the cathodes. After this period there follows a shorter time period, whereby the direction of the current is reversed (minus pulse, reverse pulse). The lengths of the pulses are typically 10-200 seconds in the plus pulse and 0.5-20 seconds in the minus pulse. The ratios for the lengths of the pulses are generally 20/1-30/1. The amperage used in a plus pulse is, by and large, greater than in a minus pulse.
The use of periodic reversal current technology is based on the decrease of the copper content of the electrolyte in the anode slime layer during the minus pulse. This prevents precipitation of copper sulfate, which is one cause of anode passivation. The decrease in copper concentration also affects the electrolyte acidity of the anode surface locally. This can further significantly affect the stability of the copper oxide coating of the anode surface, which also has a fundamental effect on the anode passivation phenomenon. With the aid of periodic reversal current technology, it has been possible to raise the current densities applied in electrolysis up to 400 A/m2 with no anode passivation.
Traditional PCR technology operates in such a way that the length of the plus and minus pulses and the amperages are set manually, after which they follow each other in a similar way, independent of the pulse of the electrolysis process. In this case, the changes in e.g. the chemical quality of the anode can bring about anode passivation at the run parameters used and especially at elevated current densities in operation. Changes in anode quality can happen very fast.
In the method according to this invention, the above-mentioned difficulty with traditional PCR technology, i.e. slowness of process control, has been overcome. According to the present invention the plus and minus pulses applied in the process do not need to be of equal magnitude, but are automatically adjusted according to the respective state in the process. The state of the process, in relation to the dissolution of the anodes, can be monitored easily and simply on the basis of electrolysis cell voltage and the current reversal can be adjusted according to this. An increase in cell voltage is a sign of incipient anode passivation and at the same time it gives an indication of the current reversal required. Accordingly, the lengths of the pulses and the amperages are no longer constant, according to the present method, but are automatically adjusted by the control logic of the power source according to the changes, particularly a rise in cell voltage, occurring in the process. The essential features of the present invention are set forth in the attached claims.
By following the changes in cell voltage of the electrolysis cells, the length of the PCR pulses and/or the amperage can be changed automatically. The cells in an electrolysis plant are normally divided into sections. Different sections can be included in the same or in different electrical circuits, depending on the number of power sources in the plant. Monitoring of changes in cell voltage can take place accordingly by cell, half-section or section or by circuit. Thus the process can be run the whole time at maximum possible current depending on the state of the process conditions.